The Pentagon says an Iranian warplane approached an unarmed U.S. military surveillance drone earlier this week over international waters in the Persian Gulf.

A U.S. spokesman said the U.S. drone - under escort by two U.S. military aircraft - was conducting routine classified surveillance on Tuesday when the Iranian plane closed to within 25 kilometers. The spokesman said all U.S. aircraft remained over international waters at all times and added that the Iranian aircraft later departed after a verbal warning.

Last November, Iran confirmed a similar Pentagon report that two Iranian warplanes fired on and missed an unarmed U.S. drone in the Gulf. Iranian Defense Minister Ahmad Vahidi was quoted at the time in official Iranian media as saying the action was ordered against an anonymous plane that had entered Iranian airspace.

Pentagon officials said the U.S. Predator drone was over international waters on November 1 conducting routine surveillance when the attack occurred.

The incident is the second such encounter made public in the past four months, and comes ahead of a visit to Israel by U.S. President Barack Obama that is expected to focus largely on Iran's suspected nuclear ambitions and Israeli vows to prevent Tehran from developing nuclear weapons.

Israel has repeatedly threatened to take military action if Iran appears on the verge of developing a nuclear bomb. The Obama administration has urged restraint and encouraged Israel to allow the pressure of international diplomacy and economic sanctions to force an end to Iran's secretive nuclear program.

Ahead of his trip, President Obama told Israeli television that it would currently take more than a year for Iran to develop a nuclear weapon.